Hi Phil,
Some of the responses you received do not answer your question.
>We are designing a Printed Board Assembly that will be required to operate
>in an ambient temperature up to 100 degrees Centigrade.
>There have been concerns expressed about the strength of solder joints at
>that temperature.
The answer is no, the strengths of solder joints, provided they are properly
wetted and not sufficiently damaged by fatigue, is never an issue. The
strength of the solder joints is adequate even in severe vibration and
mechanical shock loading. The 'stress due to thermal mismatch' is also not an
issue, since solder joint fatigue is strain-driven, not stress-driven. That
means that the total creep-yield plastic deformation in solder joints will be
the same for equal delta-Ts, but at higher Ts the creep process will be
completed faster. This has a consequence in accelerated testing, but not for
product in functional use.
As Dave Hillman pointed out however, if you cyclcle from some lower T to this
high T of 100C, you will get more creep-fatigue damage per cycle than for
T-excursions to lower maximum temperatures; thus, earlier fatigue failures
would occur in your situation. Whether this will give you adequate
reliability depends on the design and use details.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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